The present invention relates to ultrasonography, and more particularly, to a method of using pulsed-wave (PW) ultrasonography for determining an aliasing-free radial velocity spectrum of matter moving in a region. The present invention also relates to a method of using pulsed-wave (PW) ultrasonography for performing an ultrasound or medical imaging procedure on a subject.
The present invention is implementable by using any of the three main types or modalities, i.e., pulsed-wave Doppler (PWD), color flow Doppler (CFD), or tissue Doppler imaging (TDI), of pulsed-wave (PW) ultrasonography, which are used for measuring and determining radial velocity spectra of matter moving in a region. The present invention is applicable for determining an aliasing-free radial velocity spectrum of different forms, e.g., liquid or/and solid forms, of matter, moving in a (two-dimensional areal or three-dimensional volumetric) region. Such matter moving in the region is generally any substance or material, composed of organic or/and inorganic species, being of liquid or/and solid form, which is part of a non-living object, or, part of a human or animal subject. The present invention is implementable in a wide variety of different applications that are practiced in a wide variety of different fields, such as ultrasonography, medical imaging, acoustics, seismology, sonar imaging, radar technology, electronic warfare, lidar (light detection and ranging). An important exemplary application of the present invention in the fields of ultrasonography and medical imaging is echocardiography. The present invention is commercially applicable by being readily integratable and implementable with currently used pulsed-wave (PW) ultrasonography equipment and hardware (devices, apparatuses, systems).
Ultrasonography (i.e., use of ultrasound waves), pulsed-wave (PW) ultrasonography and the main modalities (i.e., pulsed-wave Doppler (PWD), color flow Doppler (CFD), and tissue Doppler imaging (TDI)), thereof, used for measuring and determining radial velocity spectra of matter moving in a region, aliasing, the theory, principles, and practices thereof, and, related and associated applications and subjects thereof, are well known and taught about in the prior art, and currently widely practiced. For the purpose of establishing the scope, meaning, and fields of application, of the present invention, following are selected definitions and exemplary usages of terminology used for disclosing the present invention.
Matter Moving in a Region
Herein, the term ‘matter’ generally refers to any substance or material which occupies space. In general, the substance or material is composed of organic species, or/and inorganic species. The matter (substance or material) can be of different forms, for example, liquid or/and solid. Moreover, the matter (substance or material) can be part of a non-living object, or, part of a human or animal subject. Herein, the term ‘region’ generally refers to any, usually continuous, segment of a surface or of a (two-dimensional or three-dimensional) space (or area). Moreover, the region can be part of a wet (i.e., liquid) or dry (i.e., gaseous (e.g., air)) environment. Accordingly, herein, the phrase ‘matter moving in a region’ generally refers to any substance or material, composed of organic or/and inorganic species, and being of liquid or/and solid form, which occupies a segment of a surface or of a (two-dimensional or three-dimensional) space (or area) that is part of a wet or dry environment.
In the fields of ultrasonography and medical imaging, particularly relevant exemplary types of matter moving in a region are blood, or tissue (e.g., muscle tissue), moving in an organ (e.g., heart, uterus, kidney, pancreas) or in some other region in the body of a (adult or fetal) human or animal subject. In the field of seismology, particularly relevant exemplary types of matter moving in a region are lava, ground water, or petroleum, moving within ground beneath the earth's surface. In the field of sonar imaging, particularly relevant exemplary types of matter moving in a region are marine vessels (e.g., submersible undersea marine vessels or craft), or marine animals (e.g., fish, whales, manatee), moving beneath the surface of (ocean or fresh) water. In the field of radar technology, particularly relevant exemplary types of matter moving in a region are vehicles (e.g., cars, trucks), marine craft (e.g., ships, boats, buoys), aircraft (e.g., airplanes, helicopters, blimps, drones, rockets, missiles), or a space craft. In the field of electronic warfare, particularly relevant exemplary types of matter moving in a region are military (land, air, marine) vehicle or craft. In the field of lidar (light detection and ranging), particularly relevant exemplary types of matter moving in a region are ground based small sized (‘hard-to-detect’) landscape objects or features (e.g., power transmission lines, rocks, roadways).
Pulsed-Wave (PW) Ultrasonography and Three Modalities Thereof.
Currently, there exist three main modalities (i.e., pulsed-wave Doppler (PWD), color flow Doppler (CFD), and tissue Doppler imaging (TDI)), of pulsed-wave (PW) ultrasonography which are used for measuring and determining radial velocity spectra of matter moving in a region, each of which is briefly defined and described hereinbelow.
Pulsed Wave Doppler (PWD) Modality
Consistent with prior art teachings [e.g., 1], herein, the pulsed wave Doppler (PWD) modality of pulsed-wave (PW) ultrasonography refers to a technique which provides information regarding the spectrum of radial velocities (i.e., the velocity component along the line-of-sight between the transducer and the region of interest) for a selected depth along a specific angular direction, as a function of time. The data is displayed as a two-dimensional graph, where the abscissa is the radial velocity (which may either be positive or negative) and the ordinate is the discrete time index. The gray level of each pixel denotes the ratio (for the scanned volume of at least part of the region) between the number of the components or elements (e.g., particles) of the matter moving at the relevant radial velocity and the total number of components or elements (particles). Thus, the outlines of the graph show the maximal velocity as a function of time. For example, in echocardiography, PWD studies are commonly used for measuring and determining radial velocity spectra of various different types and forms of matter or objects (e.g., blood, muscle tissue, valves, blood clots) moving in a cardiac muscle type of region of a subject. Two well known important applications of this technique are the evaluation of forward mitral flow (especially associated with the condition of mitral stenosis in a human subject), and the measurement of blood flow in the descending aorta (for assessing the amount of aortic regurgitation).
Color Flow Doppler (CFD) Modality
Consistent with prior art teachings [e.g., 1, 2], herein, the color flow Doppler (CFD) modality of pulsed-wave (PW) ultrasonography refers to a technique which superimposes a color representation of the mean radial velocity (for each pixel) over the two-dimensional (or three-dimensional) ultrasonic image of the matter (e.g., blood, tissue) moving in a region. The generally accepted color coding displays matter flowing towards the transducer as red and yellow hues, and matter flowing away from the transducer as blue and aqua. A well known important application of this technique is the evaluation of mitral valve regurgitation.
Tissue Doppler Imaging (TDI) Modality
Consistent with prior art teachings [e.g., 3], herein, the tissue Doppler imaging (TDI) modality of pulsed-wave (PW) ultrasonography refers to a technique which evaluates radial tissue motion in vascular and cardiac imaging. As for the CFD modality, in the TDI modality, velocity information is superimposed over a B-Scan two-dimensional image. In most relevant applications, knowing the precise temporal variability in the local signal is crucial for clinical diagnosis, so that the frame-rates used in the TDI modality are usually very high (e.g., in many cases, on the order of one hundred frames per second or more).
Brief Theoretical Basis of the Three Modalities (PWD, CFD, TDI)
All three modalities utilize the standard pulsed-wave (PW) scheme. For the pulsed wave Doppler (PWD) modality, a pulse train is transmitted in a specific direction, and the returned signal is sampled at a time slice corresponding to the relevant range. For the color flow Doppler (CFD) and tissue Doppler imaging (TDI) modalities, pulse trains are transmitted in different directions, spanning the imaging plane (or imaging volume), and the returned signal for each pulse is measured at constant time intervals, corresponding to equally spaced ranges (commonly referred to as ‘range-gates’). The radial velocity measurements are based on the Doppler effect, which (for ultrasonic imaging) are described [e.g., 2] in terms of the Doppler frequency or Doppler shift, defined by equation (1):
                              f          D                =                              2            ⁢            fv            ⁢                                                  ⁢                          cos              ⁡                              (                θ                )                                              c                                    (        1        )            
where ƒ is the transmitted frequency; ν is the absolute flow velocity; θ is the angle between the effective directions of the ultrasonic beam and the flow velocity; c is the wave speed; and ƒD is the Doppler frequency or Doppler shift, corresponding to the difference between the frequencies of the observed and transmitted ultrasound waves. The radial velocity component is ν cos(θ), therefore, the Doppler shift is directly proportional to the radial velocity of the matter moving in the region.
For each small region of interest (again, for the PWD modality there is only one), a single sample is collected for each pulse in the pulse train. Thus, the sampling interval Δ is the inverse of the pulse repetition frequency (PRF), ƒp. A straightforward method for estimating the Doppler shift is by applying Fast Fourier Transform (FFT) to the measurements (after subtracting the baseline of the frequency used for transmission). The ratio (for the relevant volume) between the number of the components or elements (e.g., particles) of the matter moving with each Doppler shift, and the total number of components or elements (particles) is simply described by the power spectrum (i.e., the element-by-element square of the FFT of the signal). For FFT, the frequency resolution, δƒD, of the output is defined [e.g., 4] by equation (2):
                              δ          ⁢                                          ⁢                      f            D                          =                  1                      N            ⁢                                                  ⁢            Δ                                              (        2        )            
where N is the number of pulses in the pulse train. The corresponding radial velocity measurement resolution, δνr, is defined by equation (3):
                              δ          ⁢                                          ⁢                      v            r                          =                                                            c                ·                δ                            ⁢                                                          ⁢                              f                D                                                    2              ⁢              f                                =                                    c                              2                ⁢                fN                ⁢                                                                  ⁢                Δ                                      .                                              (        3        )            
This method is applicable to the PWD modality, where a single region of interest is used, and hence the information for the entire relevant volume is collected in NΔ. In this configuration, N is usually 64, 128, or 256.
In the CFD modality, however, both high spatial resolution and relatively high frame rates are obtained by using very low values (e.g., 3-8) of N. The local flow velocity is coarsely approximated [e.g., 2] based on calculating the signal correlation (for each range gate in each beam position) between consecutive pulses in the train.
The configuration used in the TDI modality is somewhat more complex. For example, the wall motion of the cardiac muscle is about 10 times slower than the flow velocity of blood cells (the wall motion ranges from 0 to 0.24 m/s [e.g., 5]). Adequate frequency resolution for small N values, which are imperative for obtaining acceptable temporal and spatial resolution, requires using relatively low PRFs. This, in turn, reduces either the spatial or the temporal resolution of the collected data. In order to support the high frame rates necessary for the TDI modality, only a small number of beam positions are scanned at each frame (about 16). The radial velocity is estimated using either short FFTs or correlation techniques.
Aliasing Associated with Pulsed-Wave (PW) Ultrasonography, and Problems Thereof.
All three imaging modalities, PWD, CFD, and TDI, of pulsed-wave (PW) ultrasonography suffer from the same major limitation: aliasing. Consistent with prior art teachings [e.g., 4], herein, the term aliasing refers to an artifactual type of phenomenon that arises when the Nyquist critical frequency is exceeded by at least one frequency component of the input signal.
For any sampling interval Δ (corresponding to a PRF ƒp=Δ−1), there is also a special frequency ƒc, called the Nyquist critical frequency, defined [e.g., 4] by equation (4):
                              f          c                =                  1                      2            ⁢                                                  ⁢            Δ                                              (        4        )            
When sampling a signal, that is not bandwidth limited to the range between the negative Nyquist critical frequency and the positive Nyquist critical frequency, [−ƒc, ƒc], any frequency component outside these limits is falsely translated, or aliased, into that range, by the very act of discrete sampling. Note that, unlike the frequency resolution (equation (2)), the Nyquist frequency is independent of the length of the pulse train N.
Furthermore, particularly applicable to pulsed-wave (PW) ultrasonography, is that the Nyquist frequency is inversely proportional to the maximal penetration depth R, defined by equation (5):
                    R        =                              c            ⁢                                                  ⁢            Δ                    2                                    (        5        )            It is assumed that each pulse is transmitted only after reception of reflections from the furthest relevant range of the previous pulse.
In the field of radar technology, when there exists the possibility for aliasing to occur during Doppler measurements, the Doppler measurements are considered as being ambiguous, i.e., characterized by ambiguities.
Prior Art Techniques for Addressing Aliasing, and Limitations Thereof.
Until now, several techniques for bypassing, overcoming, or reducing (suppressing), the undesirable effects of aliasing have been suggested. Fundamental methods of such techniques [e.g., 6, 7, 8] are particularly applicable to the CFD modality of pulsed-wave (PW) ultrasonography, and are based on tracking the mean Doppler frequencies immediately before and immediately after the Nyquist frequency, either along the temporal axis or along the spatial axis. Such techniques are limited by being relatively noise-sensitive (i.e., strong noise may cause false alarms in the identification of aliased signals), and may only double the maximal unambiguous frequency (i.e., the maximal ‘true’ frequency at which aliasing is absent).
There is a technique [9] which is applicable to the PWD modality of pulsed-wave (PW) ultrasonography, that enables measurement of frequencies exceeding the Nyquist frequency, so long as the total bandwidth of the measured signal is less than, or equal to, the pulse repetition frequency (PRF). This technique simply varies interpretation of the measured spectral pattern, by moving the center Doppler frequency of the analyzed band from zero to the instantaneous mean Doppler frequency, thereby avoiding the aliasing effect on the displayed signal.
A more complex technique exists [10] which is designed for suppressing aliasing during PWD imaging. This technique involves summing the signal along skewed lines in the time range plane (that is, prior to applying FFT), where the slope is chosen to follow the movement of the reflectors or scatterers along the ultrasonic beam for each velocity in the velocity spectrum. Only when the slope matches the actual velocity do the received echoes match in phase and amplitude, giving a peak in the spectrum at the actual velocity value. This concept is implemented by performing integration over the two-dimensional power spectrum of the time range signal, weighted by a velocity dependent spectral window function.
The double repetition-rate technique, which enables the estimation of Doppler spectra (for the PWD modality) with a bandwidth higher than the pulse repetition frequency (PRF), was first introduced by Newhouse et al. [11]. This technique is based on the observation that the measured Doppler frequency for aliased signals depends on the transmitted PRF, whereas the measured Doppler frequency for true, non-aliased signals, is independent of the PRF. Thus, if a system uses two alternating PRFs, then, comparing the two spectra taken at the two PRFs enables identification of aliased frequency components, i.e., the aliased frequency components are displaced relative to zero frequency in accordance with the PRF, while the non-aliased spectral components are not displaced.
A main limitation of the double repetition-rate technique is that the maximal unambiguous frequency (i.e., the maximal ‘true’ frequency at which aliasing is absent) is, at best, increased by a factor of 2. Additionally, this technique is only applicable when the Doppler frequency peaks are relatively narrow and easily discernable (resolvable) from each other. However, it is well known [e.g., 11] in clinical practice that many high velocity flows, such as during mitral stenosis, are turbulent, and thus generate Doppler spectra characterized by relatively broad and non-discernable (non-resolvable) peaks.
Another technique [12] which is particularly applicable to the PWD modality of pulsed-wave (PW) ultrasonography utilizes two ultrasound carrier frequencies, such that an appropriate processing of the measured Doppler frequencies results in an extended range of mean velocity measurement of matter moving in a region. As understood from equation (1), the carrier frequency also affects the Doppler shift, so that the outcome of using two different carrier frequencies is similar to that of using two pulse repetition frequency PRFs. With the extended mean velocity information, the complex Doppler signal is interpolated to reconstruct the aliased Doppler spectra. Such a technique is useful, however, it is also limited in that interpolation inherently introduces inaccurate information into the imaging system. These inaccuracies become severe when the data region includes several dominant velocities, whereas the interpolation scheme assumes a specific mean frequency. This is the case in the presence of several simultaneous jets within the region of interest, for example, due to a combination of mitral valve regurgitation and a shunt between the left ventricle and the right ventricle.
In radar technology, there are teachings [e.g., 13] of a basic technique for bypassing, overcoming, or reducing (suppressing), undesirable effects of Doppler measurement ambiguities (analogous to occurrence of aliasing in pulsed-wave (PW) ultrasonography). The technique is based on transmitting pulses at two alternating pulse repetition frequencies (PRFs), and leads to evaluating the dominant Doppler frequency corresponding to a very narrow velocity spectrum corresponding to an airborne target. Historically, this technique has never been suggested, thought, or considered, as being applicable to addressing the problem of aliasing in pulsed-wave (PW) ultrasonography. This technique is analogous to bypassing, overcoming, or reducing (suppressing), the undesirable effects of aliasing in the color flow Doppler (CFD) and tissue Doppler imaging (TDI) modalities of pulsed-wave (PW) ultrasonography.
Accordingly, based on the preceding discussion, prior art techniques for addressing aliasing which occurs when using pulsed-wave (PW) ultrasonography for determining radial velocity spectra of matter moving in a region are significantly limited.
There is thus a need for, and it would be highly advantageous to have a method of using pulsed-wave (PW) ultrasonography for determining an aliasing-free radial velocity spectrum of matter moving in a region. There is also a need for, and it would be highly advantageous to have a method of using pulsed-wave (PW) ultrasonography for performing an ultrasound or medical imaging procedure on a subject.
There is also a need for such an invention which is implementable by using any of the three main types or modalities, i.e., pulsed-wave Doppler (PWD), color flow Doppler (CFD), or tissue Doppler imaging (TDI), of pulsed-wave (PW) ultrasonography, which are used for measuring and determining radial velocity spectra of matter moving in a region.
There is also a need for such an invention which is applicable for determining an aliasing-free radial velocity spectrum of different forms, e.g., liquid or/and solid forms, of matter, moving in a (two-dimensional areal or three-dimensional volumetric) region, where such matter moving in the region is generally any substance or material, composed of organic or/and inorganic species, being of liquid or/and solid form, which is part of a non-living object, or, part of a human or animal subject. Moreover, there is need for such an invention which is implementable in a wide variety of different applications that are practiced in a wide variety of different fields, such as ultrasonography, medical imaging, acoustics, seismology, sonar imaging, radar technology, electronic warfare, lidar (light detection and ranging). Furthermore, there is a need for such an invention which is commercially applicable by being readily integratable and implementable with currently used pulsed-wave (PW) ultrasonography equipment and hardware (devices, apparatus, systems).